What’s the deal with Diesel Exhaust Fluid (DEF)?

Many diesel vehicles, such as Mercedes' BlueTEC models, require an exhaust additive.

Mercedes-Benz

Drivers are required to refill their auto’s DEF tank every 4,000 to 10,000 km, depending on vehicle use and load – but choosing the wrong supplier can have dire consequences

by
Brian Turner | March 9, 2017

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Regardless of the Volkswagen diesel emissions scandal that’s still reverberating in government houses and law courts around the globe, diesel engines have made inroads into light trunk and passenger vehicle sales. What more and more drivers are learning about every day is one of the emissions control features of post-2009 model year diesels: Diesel Exhaust Fluid, or DEF.

DEF and its sophisticated control/delivery systems are stored on board to inject this urea-based fluid into the exhaust stream to help lower NOx tailpipe output. Drivers are required to refill their auto’s DEF tank every 4,000 to 10,000 km, depending on vehicle use and load. DEF composition should be near 32 per cent automotive grade urea and the remainder purified water. An instrument panel display will provide monitoring of the distance remaining to refill time. If the tank is depleted, the vehicle may refuse to restart after a shut-down, making the DEF gauge/monitor something a diesel driver doesn’t want to ignore.

One area of concern for many light-vehicle diesel owners is the major cost spread for DEF fluid. Original automaker dealerships will charge US$12.00 per gallon and up, and depending on the distance a vehicle travels (and other engine demands), the average driver can go through several gallons per month or more. But DEF is also used in heavy trucking (with this use predating passenger vehicle or light truck applications), which is why almost all commercial truck refuelling centres offer bulk DEF dispensed through fuel-pump type stations. These DEF retailers can offer prices as low as US$3.00 per gallon.

Like heavy trucks, passenger vehicle DEF systems continually monitor (and report via the instrument panel) the operation of this emission control feature, including the ability to warn of poor-quality DEF, which may or may not reduce engine power to limit NOx emissions until the problem is resolved. In the commercial world, this is known as “de-rating” the engine and it can make hauling 186,000 lbs of freight up a steep, winding road a white-knuckle affair. In many cases, if a “DEF quality” warning is activated, the system’s sensors or software can just as often be responsible for the fault as less-than-stellar-grade DEF. But when you take your vehicle to a repair centre, and it’s out of warranty (or sometimes even within the terms), be prepared to be charged for the diagnosis and costs of draining and refilling the DEF tank, along with any other repair requirement found by the tech.

Large-volume, national-brand commercial refuellers have a lot at stake when it comes to ensuring DEF quality, as they can be held liable for the expenses of heavy truck diagnoses and repairs which are shown to be caused by poor-quality DEF (not to mention the costs of freight delays). But regardless of the reputation of a highway DEF retailer, or the problems a particular make and model may have with DEF operation, automaker dealership service departments will usually stand by their position that only their authorized DEF fluid should always be used if the system’s warranty is to remain valid.

If your diesel’s dash pops up a DEF warning, you should first ask your service garage to check for any manufacturer bulletins regarding upgraded software or DEF components that may be related to whatever fault code is stored. Remember, if the fault is traced to impure DEF fluid not sourced from the automaker’s approved supply, you may face diagnostic fees when the vehicle is still within warranty limits. Sometimes draining and refilling the DEF tank from a confirmed good source may rectify the fault, but it’s not always an easy DIY task. If your ride has had a history of repeated DEF quality faults, you may want to invest in a DEF tester tool (available from many auto-parts stores); it’s a refractometer than can quickly verify DEF condition with a few drops of the liquid. Or better yet, if you can connect DEF dash warnings to particular DEF suppliers, avoid those in the future. Saving a few dollars a month in DEF expenses is quickly eclipsed by the cost of diagnostics and repairs.